1. Field of the Invention
The present invention is related to a ceramic heater and, more particularly, to a ceramic heater suitable for use in a diesel engine glow plug.
2. Discussion of the Related Art
Restrictions on diesel engine emissions, which can be expected to become ever more stringent, are more easily satisfied by increasing the temperature of the engine's glow plugs, which significantly enhances the engine'cold start characteristics and reduces the generation of exhaust gases such as white smoke. Ceramic heaters, formed by embedding a heating element consisting of a refractory metal (typically a tungsten wire) within a body consisting of a sintered, silicon-based ceramic (typically silicon nitride), are generally used in glow plugs. The durable ceramic heaters thus formed are resistant to both corrosion and thermal shock in high-temperature ambients (the maximum temperature at the surface of a glow plug ceramic heater in a typical diesel engine is roughly 900.degree. C.).
Should such a ceramic heater be sintered at temperatures of 1600.about.1800.degree. C. during formation or exposed to surface temperatures in excess of 1300.degree. C. during operation (as would be the case in a gas injection diesel engine), the operational characteristics of the ceramic heater would suffer due to chemical reaction between the silicon nitride body and the tungsten of the heating wire. Tungsten consumed by the formation of tungsten silicide (WSi.sub.2) at the interface of the wire and the body would locally reduce the cross sectional area of the wire. Local decreases in the area of the heating element would in turn cause local increases in resistance and the resultant local overheating would cause premature rupture of the tungsten heating element. Conventional ceramic heaters are thus not used at temperatures much in excess of 900.degree. C.